Remove the need for HostRef<Store> (#771)

* Remove the need for `HostRef<Store>`

This commit goes through the public API of the `wasmtime` crate and
removes the need for `HostRef<Store>`, as discussed in #708. This commit
is accompanied with a few changes:

* The `Store` type now also implements `Default`, creating a new
  `Engine` with default settings and returning that.

* The `Store` type now implements `Clone`, and is documented as being a
  "cheap clone" aka being reference counted. As before there is no
  supported way to create a deep clone of a `Store`.

* All APIs take/return `&Store` or `Store` instead of `HostRef<Store>`,
  and `HostRef<T>` is left as purely a detail of the C API.

* The `global_exports` function is tagged as `#[doc(hidden)]` for now
  while we await its removal.

* The `Store` type is not yet `Send` nor `Sync` due to the usage of
  `global_exports`, but it is intended to become so eventually.

* Touch up comments on some examples

* Run rustfmt

crates/misc/rust/macro/src/lib.rs

@@ -52,8 +52,8 @@ fn generate_load(item: &syn::ItemTrait) -> syn::Result<TokenStream> {
             use #root::anyhow::{bail, format_err};
 
             let engine = Engine::new(Config::new().wasm_multi_value(true));
-            let store = HostRef::new(Store::new(&engine));
-            let global_exports = store.borrow().global_exports().clone();
+            let store = Store::new(&engine);
+            let global_exports = store.global_exports().clone();
 
             let data = #root::wasmtime_interface_types::ModuleData::new(&bytes)?;